undergone by its Salts at High Temperatures. 307 



law holds good in part with many organic acids; some of 

 these acids requiring two, and some three atoms of base, to 

 form neutral salts. The hydrates of each acid contain a 

 corresponding number of atoms of basic water, which cannot 

 be removed without the decomposition of the acids them- 

 selves. These chemists showed that, though a salt of any 

 acid, with magnesia or with oxides isomorphous with it, pos- 

 sessed the power of combining with a potash salt of the same 

 acid, and forming a double salt, such as the sulphate of mag- 

 nesia and potash, that is not ground for doubling the atomic 

 weight of the acid, or for viewing it as bibasic. 



They proved at the same time that a monobasic acid is 

 incapable of forming a double salt with two isomorphous 

 bases. The proportion of base which unites with a poly- 

 basic acid is constant, generally either two or three atoms. 

 In the memoir on organic acids by Liebig, here alluded to, 

 he had made it not improbable that malic acid is bibasic. At 

 his suggestion I have made several analyses of its various salts, 

 which form the subject of this paper. 



Malic acid was first discovered by Scheele in the juice of 

 the apple; it was again discovered by Donovan * in the juice 

 of several plants, and described by him as a new acid. The 

 identity of the acid of Donovan with malic acid was proved 

 by Braconnot. This acid has been most fully described by 

 Liebig. It has also been partially examined by Pelouze, 

 Braconnot, and Richardson. 



The malic acid used in the present investigations was pre- 

 pared from the expressed juice of the berries of the Sorbus 

 aucuparia, or Service tree, in the following manner. The ex- 

 pressed juice was mixed in a copper pan with finely divided 

 and levigated hydrate of lime, care being taken not to satu- 

 rate the fluid completely, but to allow it to remain sensibly 

 sour. Being placed on the fire, it was made to boil for some 

 hours, during which time it gave off a peculiar pungent 

 vapour, which strongly affects the eyes. By degrees neutral 

 malate of lime precipitates, and may be removed with a ladle. 

 By continued boiling, more of the salt is obtained. When 

 no more falls, the vessel is removed from the fire, and allowed 

 to cool, when a little more is precipitated. We must take 

 care in the beginning not to saturate the expressed juice en- 

 tirely with lime, or so much colouring matter falls with the 

 malate of lime, as to render the acid impure. The neutral 

 malate of lime thus obtained, is dissolved in dilute nitric acid 

 (1 part acid to 10 of water), filtered and evaporated; upon 



[* Mr. Donovan's paper was reprinted from the Phil. Trans, in Phil. 

 Mag., first series, vol. xlvi. p. 433.] 



X2 



